Removable mat-based insulations made up of the following materials are known:
1. A glass fiber fabric with a stainless steel foil or sheet on its outer face as the encasement of the inner face.
2. A 1 inch (25 mm) thick ceramic fiber insulation layer with a density of 8 lb/ft3 (128 kg/m3) as inner insulation.
3. A glass fiber fabric coated with silicone or Teflon as the encasement of the outer face (depending on the engineering).
4. These mats were sewn at all their ends by a glass fiber wire resistant to high temperatures.
Theoretically, these specified mats met some of the requirements that were specified, and they did not meet others but it was considered the only solution. These requirements were the following:
A. In the event of a thermal oil leak in the attachments between pipe segments in the swivel arms, which were already provided for, the oil could not impregnate the insulation whereby the outer face of the encasement of the inner face of the mat could not let the oil pass through to the mats covering the three joints of each arm. This was achieved with the glass fiber fabric with the stainless foil because the stainless acted as a barrier for the oil.
B. The insulation in the rotating joints had to be easily removable for frequent required maintenance to graphitize the rotating joints. This was achieved by making independent mats for the rotating joints.
C. The outer encasement of the mat had to be impermeable to prevent the insulation from getting wet because this would cancel out the insulating effect and would furthermore progressively deteriorate it. This was achieved with the silicon or Teflon fabric.
D. The arm had to be perfectly insulated and, like the rest of the pipes, had to have minimal heat losses. This was not achieved because the 25 mm layer of ceramic fiber achieves only a 23-29% insulating power with respect to the 100 mm thick mat of 100 kg/m3, which is the material and thickness used for insulating pipes up to 3″ in diameter conducting heat transfer fluid in the solar field because the lambda values thereof at 400° C. are very similar. Thicker ceramic fiber could not be installed because the pipe of the arms is 2″ or 2.5″ in diameter and it is impossible to manufacture mats for the elbows and for the ball joints such that they correctly close and absorb, without leaving ball joints, the movements of the arm. It was considered that there was not a better solution and the mats were installed.
E. The insulation-encasement system installed had to absorb the movements of the swivel arm, in all its directions, without causing the insulation to slide, the encasement to deteriorate and, accordingly, without forming rotating joints through which heat losses occur and water can enter. Since the mats are installed in new condition, they meet this requirement, but experience in several plants has demonstrated that since they are mats which are tied with wire or with glass fiber fabric strips, with the continuous arm movement and the action of atmospheric agents, sliding takes place and rotating joints are formed, all of which completely deteriorate the quality of the insulation.
After about 7-12 months of the mats being installed in the Spanish plants, several problems were detected and, when communicating with the American plants, it was corroborated that new problems had arisen that were not foreseen during the design stage and that the mats furthermore did not meet these requirements. These problems were the following:
F. The ray of sunlight reflected by the mirrors on the pipe is reflected not only frontally towards the pipe conducting the heat transfer fluid but it is also reflected laterally, and depending on the time of day or season of the year, there are concentrations of solar radiation on the mats of up to 80 kW/m2. This means that the silicone or Teflon is withstood at very high temperatures, the engineering firms estimate that between 350° and 450°, and after 140-180°, at the most, these materials disintegrate causing the glass fiber, which allows the entrance of water, to gradually wizen and the mats open up and end up breaking and falling.
G. In two of the Spanish plants, there were heat transfer fluid leaks in rotating joints, leading to ignition and causing considerable fires in the area of the rotating joint. After investigating the causes of this ignition, when the insulating materials and the stainless foil are fire-resistant, it was concluded that it was because the thermal oil at 400° under the pressure at which the fluid is conducted and in contact with oxygen can cause ignition if that pressure is contained within the mat and does not find an escape route. With the mats this pressure has no escape route, so the risk of ignition of the heat transfer fluid is highly probable. The fires occurring in the Spanish plants in the rotating joints were one of the biggest engineering problems because the environmental hazard is very considerable if the fire spreads.
H. The swivel arms are supported in most engineering designs by frames with rotating tubes acting as bearings so that the arms move through those bearings. Since the ceramic fiber-insulated mats lack rigidity, they gradually crush the insulating material, breaking it and moving it to the sides, whereby finally the area of the mats moving through the supports virtually ends up with no insulation with the subsequent losses.
The mats have been deteriorating little by little in all the facilities and a patch has been made in most consisting of deflecting collars placed above the mats and preventing the solar concentration thereon. This solution has two drawbacks, the first being that it makes the solution considerably more expensive, and the second being that these deflectors have a sail effect and distort the calculated movement of the mirrors, whereby being able to result in breakdowns in the rotating mechanisms.